Stereotactic radiosurgery (SRS) has been traditionally used to treat brain disorders such as brain tumors and lesions with a precise delivery of a high dose of radiation. Focused radiation beams are delivered to a specific area of the brain, in essence burning tissue to treat abnormalities, tumors or functional disorders. The radiation is often applied in a single dose during a one-day session. Single-session radiosurgery normally has such a dramatic effect in the target zone that it is as if the tissue were removed, and thus the changes are considered “surgical.”
Radiosurgery historically began by treating targets in the head and neck, because these areas can be immobilized with a skeletal fixation device that completely restricts the head's movement, permitting the most precise and accurate treatment. One-session treatments without such a skeletal fixation device have not been recommended because of the high potential for damage to healthy brain tissue, cranial nerves (optic, hearing, etc.) and the brain stem.
The clinical targets for radiosurgery are generally relatively small and well defined. High-resolution 3D imaging techniques such as CT and MRI help identify and clinically define these targets and the critical structures surrounding them. Through the use of three-dimensional computer-aided planning and the high degree of immobilization, stereotactic radiosurgery attempts to minimize the amount of radiation that passes through healthy brain tissue. This may be the primary treatment, used for example when a tumor is inaccessible by surgical means, or as a boost or adjunct to other treatments for a recurring or malignant tumor, although in some cases, radiosurgery may be inappropriate.
Stereotactic radiosurgery plans are often done using cone arcs, a well established technique in which open-ended metal cones are placed over one or more radiation sources. Cone arcs result in ball-shaped regions of high intensity radiation near isocenters that provide sharp dose gradients around target regions. For this reason, cone arc therapy is well suited for circular or spherical regions, but more difficult to use for irregular areas.
However, cone arc therapy is generally done mostly manually, relying on the skill of the user, i.e., the radiosurgeon. While a radiation source may be moved in an arc to match the outline of the target region, the user adjusts the isocenters manually. There is generally no shaping of the radiation beam other than to create the circular isocenters, and thus with limited exceptions there are no other shapes such as might be created using a leaf shutter as in radiation therapy. It is believed that there is also no dose distribution-based optimization of the radiation, other than adjusting the size of the isocenters.
While trajectory and arc treatments with multi-leaf collimators have proven efficient and practical in other fields of radiation therapy they are not yet widely used in radiosurgery. Thus, optimization methods that are specially designed to meet radiosurgery needs have not been available for intensity modulated trajectory and arc treatments. For example, in complex cases the treatments may require more isocenters, while a low number of isocenters is preferred due to shorter treatment times.
Fractionated stereotactic radiation treatments, in which the radiation dose is received over a period of days or weeks, are sometimes used with the assistance of removable masks and frames. These devices generally achieve a lesser degree of immobilization than those used in single-session radiosurgery, and thus increase the risk of unintentionally exposing healthy tissue to radiation. Thus, as with single session radiosurgery, body radiosurgery using fractionated treatments is rare, as it is difficult to adequately immobilize and treat the body, although such treatment is becoming more common, particularly for targets in the spine and other extracranial organs. Recent studies have also suggested that this strategy can be more effective at killing or controlling certain types of cancer.
By contrast, external beam radiotherapy, often simply referred to as radiotherapy, is a radiation delivery procedure that generally uses a number of dose fractions, as many as 30 or more, of low dose high-energy radiation. Radiotherapy is usually administered over a period of weeks, and is typically used for larger tumors, a larger number of tumors, for end-stage disease tumors in combination with chemotherapy, and for systemic diseases such as blood-borne cancers. The goal of radiotherapy is tumor control or disease palliation; it is sometimes said to operate under the radiobiological assumptions of the “Four Rs,” reoxygenation, reassortment, repopulation, and repair.
In radiotherapy, the goal is typically different than that in radiosurgery; rather than a dose sufficient to kill all tissue in the target area, radio therapy seeks to obtain a flat, homogenous and continuous dose across the target area, with a low dose in the surrounding areas. This is typically accomplished by intensity modulated treatments which optimize the radiation from different directions, often with the use of a leaf shutter on the radiation source that may be adjusted so that a different shape or cross-section is projected onto the patient from each direction where the radiation source is activated.
In other prior art methods dose-volume histogram (DVH) objectives, monitor unit (MU) objectives and normal tissue objectives have been used to optimize intensity modulated trajectory treatments. It is believed that DVH data has even been occasionally used with leaf shutters in radiosurgery. But DVH data has no spatial data, making it more complex to get plans that meet the necessary objectives typical for radiosurgery treatments.
For these and other reasons, techniques used in radiosurgery are generally not used in radio therapy, and vice versa. It would be useful to have an intensity modulated trajectory and arc optimization method that allows efficient optimization of properties that are important in radiosurgery applications.